WHAT! I thought you were going to advise ME!
I may do a better job of confusing you than advising you.
All the previous comments on this problem are reasonable, but I keep coming back to source of the problem being water quality. I am not referring to water quality based on the usual parameters, but on the broader definition that includes DOM (Dissolved Organic Matter), BOD (Biological Oxygen Demand) among others
and the fact that parameter levels will vary dependent on location in the water body. Oxygen content, for example, will be higher at the base of a waterfall than at the very bottom of any pond.
It is because of these variances in water parameter levels due to immediate environmental factors that my thoughts keep returning to your 'bog'.
Best case scenario is that the pathogens contained in the turtle feces (there are pathogens in all vertebrate feces) are being carried to the pond proper by the planned water flow. Worst case scenario is that you do not have a completely diffused water flow in your 'bog'. Contrary to what others may say, 'bogs' will clog and form 'dead spots' where water will not penetrate but will flow (Channel) around the obstructed area. This results in anaerobic conditions within these 'dead spots' which can become a proverbial nursery for pathogenic bacteria.
Does your 'bog' exhibit an even water flow across its entire surface or are there areas where water flow is considerably more noticeable.
The point that I am trying to make is that water quality in the main pond basin may test acceptable but only a few feet or even inches away a veritable time bomb may be ticking in your 'bog'.